1. Field of the Invention
The present invention relates to a method for preparing alkali cellulose and a method for preparing cellulose ether by using the alkali cellulose thus obtained.
2. Description of the Related Art
For the preparation of cellulose ether, known is a method of bringing an alkali solution into contact with a highly purified pulp to prepare the corresponding alkali cellulose, and etherifying the resulting alkali cellulose with an etherifying agent. The cellulose ether obtained as a final product becomes water-soluble by properly controlling its degree of substitution. The water-soluble cellulose ether contains a water-insoluble portion which sometimes lowers the light transmittance of the aqueous solution of the cellulose ether or damages the commodity value of the cellulose ether as a contaminant. This insoluble portion appears to be caused by the presence of a low-substituted portion which does not have enough substituents to dissolve in water. One of the reasons why the insoluble portion is present is that an alkali distribution is uneven in the alkali cellulose.
Functions of this alkali include swelling the cellulose therewith to change its crystal structure in the pulp and thereby promoting the penetration of the etherifying agent; catalyzing the etherification reaction with an alkylene oxide; and serving as a reactant for an alkyl halide. A portion of pulp not brought into contact with the aqueous alkali solution is not involved in the reaction and therefore remains as an undissolved portion. Lack of uniformity of the alkali cellulose directly leads to the undissolved portion.
A widely used method for preparing alkali cellulose includes a method comprising the steps of adding an alkali to a powdery pulp, which has been obtained by pulverizing a pulp, in an amount just necessary for the etherification reaction, and mechanically mixing them (Japanese Patent Application Examined Publication No. 60-050801/1985 and Japanese Patent Application Unexamined Publication No. 56-002302/1981). In this method, however, the alkali is not distributed uniformly in the pulp. The pulp not brought into contact with the alkali fails to become cellulose ether and remains in the final product as an unreacted portion, thereby lowering the quality of the cellulose ether.
In order to prevent such a problem, as described in Japanese Patent Application Unexamined Publication No. 60-040101/1985, proposed is a method comprising the steps of immersing a pulp sheet in an excess alkali solution to allow the pulp to absorb the alkali sufficiently and then pressing the resulting pulp under pressure to remove an excess alkali over the intended amount of alkali. When this method is industrially applied, it is typical to pass a supporting shaft through a center tube of pulp wound into a roll form, and elevate the rolled pulp, thereby lifting it up from a floor surface, or place the pulp wound into a roll form on a skid, to allow the pulp to rotate freely; and introduce the pulp into an immersion tank while unwinding the pulp into the pulp sheet. In this method, however, it sometimes becomes impossible to continue an operation because the pulp sheet is pulled and broken during immersion. In addition, mass production requires a very large immersion tank for immersing the pulp sheet therein for a certain time, which is disadvantageous because a sufficient space for installing such a tank cannot be secured easily or an investment cost becomes large. On the other hand, when the pulp is in chip form, pressing becomes uneven because of lack of flat surfaces in the obtained cake so that the alkali distribution in the alkali cellulose becomes uneven, leading to quality deterioration.
On the other hand, in page 433 of “Cellulose No Jiten (Encyclopedia of Cellulose)” edited by the Cellulose Society of Japan and published on Nov. 10, 2000, described is a method for preparing alkali cellulose with respect to viscose preparation, the method comprising the steps of adding a pulp to an alkali solution to form a gruel-like slurry and pressing the slurry by a slurry press. Although some problems caused by use of pulp sheet can be solved, the slurry press causes uneven pressing so that an alkali distribution in the alkali cellulose becomes uneven, leading to the lowered quality. In addition, alkali cellulose having relatively small alkali content, which is required as a raw material of cellulose ether, is difficult to be prepared by this method alone because of limitation in the pressing performance. Thus, it is difficult to apply this method to the preparation of cellulose ether.
In Japanese Patent Application Examined Publication No. 03-073562/1991, described is a method for preparing cellulose ether having a desired composition, comprising the steps of preparing alkali cellulose from cellulose and excess alkali, and washing the alkali cellulose with a hydrophilic solvent to remove the alkali. However, this method requires large scale equipment and many steps. In addition, the hydrophilic solvent remains in the alkali cellulose and causes a side reaction with an etherifying agent so that efficiency of the reaction with the etherifying agent decreases. Further, this method requires neutralization of the washing liquid or recovery of the alkali so that it is not suited for the industrial production of cellulose ether.